Characterization of Pre- and Post-Service Grain Boundary Phases in a Cast Austenitic Steel

摘要

Austenitic steel castings are currently being used in components for industrial gas turbine engines. Service experience has indicated a degradation of mechanical properties with extended exposures at elevated temperature. The purpose of this study was to characterize the grain boundary phases that develop during the casting processes as a likely explanation for the observed performance. In order to isolate these precipitates, a variety of electron microscopy characterization techniques were used to characterize their composition after various heat treatments and service exposure. In the baseline, as-cast and annealed condition, a discontinuous network of grain boundary metal carbides was observed. These precipitates coarsened during short-term annealing at 649°C and a denuded zone formed in the adjacent matrix. When the 38,600 h service-exposed material was analyzed, the grain boundaries were highly decorated with a more continuous film of grain boundary carbides as well as voids attributed to creep cavitation. In addition to carbides, acicular A1N precipitates were identified on the grain boundaries of the casting examined after service exposure.